Coin batteries, also known as flat batteries, are small in size and thickness, and because of this feature, are widely used for watches, keyless entries and other applications that require miniature power sources. Coin batteries are also used in memory backup for office automation (OA) devices and factory automation (FA) devices and other applications that require power sources with long operating life, and as power sources for various meters and measuring devices, and the range of application thereof is expanding. The operating environment of coin batteries is also expanding from room temperature environment to low temperature or high temperature environment.
As mentioned above, with changing circumstances surrounding coin batteries, the demand for improvement in quality of coin batteries is getting severer. There also is a stronger demand for lower price of coin batteries.
A coin battery is composed of a power generation element and a housing accommodating the power generation element. The housing includes a shallow cylindrical battery case, a sealing plate closing the opening of the battery case, and a gasket interposed between the opening end of the battery case and the rim of the sealing plate. A positive electrode and a negative electrode are disposed face to face with a separator interposed therebetween in the battery case, and an electrolyte is injected thereinto. The opening end of the battery case is then crimped onto the rim of the sealing plate, with a gasket interposed therebetween. In such a way, the interior of the housing is hermetically closed.
For the sealing plate and the battery case, stainless steel has been conventionally used. Stainless steel has high mechanical strength, and therefore, the battery case can be firmly crimped onto the rim of the sealing plate with less deformation, and the power generation element can be enclosed with excellent hermeticity.
There is recently proposed a technique of using a nickel-plated steel material, which is less expensive than stainless steel, as a material for the sealing plate. Specifically, it is reported that, by controlling the ratio of a curvature radius R of a curved portion formed around the periphery of the top of the sealing plate to an interior angle θ of the curved portion, the deformation of the sealing plate when processed can be suppressed, and the occurrence of rust on the outer side of the sealing plate and electrolyte leakage can be suppressed (see Patent Literature 1).